(a) Field of the Invention
The present invention relates to an objective for endoscopes and, more particularly, to a retrofocus-type objective comprising a front diverging lens group and a rear converging lens group.
(B) Description of the Prior Art
Most of conventional objectives for endoscopes comprised only one, two or three lenses since it was rather difficult to machine and assemble plural lenses having diameter small enough to be accommodated in this distal ends of endoscopes. Of the rays incident on an image fiber bundle, on the other hand, those having large angles of incidence can not reach the other end of the image fiber bundle due to optical leakage from the image fiber bundle. In order to minimize such optical leakage of the incident rays, it is therefore necessary to make the incident rays as parallel as possible. For achieving this purpose, a stop is arranged in front of the objective when an objective comprising only one lens is adopted for endoscopes. Such a stop is also employed for objectives for endoscopes even when they comprises two or three lenses.
In case of endoscopes for side viewing, a prism is placed in front of the stop which is arranged in front of the above-mentioned objective. When a wide angle of field is desired, however, the prism must unavoidably have dimensions too large to be arranged in a thin distal end of endoscopes.
Further, a large prism will inevitably require a large cover glass consequently.